I propose to study functional development of B lymphocyte subpopulations responding to thymus dependent and thymus independent antigens in vitro. These subpopulations have been shown to exist in both the primary and memory B cell pools in the mouse for the trinitrophenyl (TNP) hapten. They have also been shown to exist in the primary pool for the phosphorylcholine (PC) hapten. I propose to demonstrate their existence in PC memory pool. We have developed a model for memory to PC expressed in all three major classes of immunoglobulin--IgM, IgG and IgA. Preliminary evidence indicates that the anti-PC memory response is more heterogeneous than the primary response and that the order of heterogeneity in the memory response is IgG greater than IgA greter than IgM with the latter still composed mainly of T15-idiotype positive cells. I proposed to examine the heterogeneity of the anti-PC memory response by idiotype analysis, avidity of the plaque-forming cell response and by isoelectric focusing. The role of regulatory T cells in controlling the expression of heterogeneity in the memory pool will be examined. We shall also compare PC-specific IgA memory B cells from the systemic (spleen) and mucosal (mesenteric lymph nodes) lymphoid system for the degree of heterogeneity developed in the two systems. We shall test the hypothesis that similar subsets of functionally distinct B cells occur in the human lymphoid system. We have already established that human lymphocytes can give rise to thymus dependent and thymus independent anti-TNP responses in vitro. The existence of these subpopulations will be examined by additivity in responses to thymus dependent and thymus independent B cells, limiting dilution analysis of precursor frequencies and independent killing by BUdR and light. The heterogeneity of the subpopulations will be examined by plaque inhibition and by comparative susceptibility to tolerance induction.